Spark gaps can operate as switches to control the flow of very large electrical currents under very high voltage conditions. Typical applications include accelerators, high-power gas laser systems, and other pulsed power systems. Spark gaps operate to prevent the flow of electrical current in high voltage applications by filling the space between a pair of electrodes with an insulating gas. The electrodes are held in fixed position relative to one another by a solid dielectric material that is part of the spark switch walls.
When current flow is desired, a trigger pulse on an intermediate electrode or some other means is used to locally change the state of the insulating gas and thus create a more conductive path. The lower resistance locally leads to a rapid breakdown of the gas between the electrodes at approximately the hold-off voltage. The breakdown very rapidly produces a very low resistance conduction path, i.e., a spark or arc, through the insulating gas and transfers a high energy electrical pulse to a load.
Losses in the spark release large amounts of energy in various forms during each pulse. Besides the light energy produced by the spark, significant amounts of thermal energy are also produced. In addition, the rapid temperature and pressure change of the gas in the spark results in a series of very strong shock and expansion waves that propagate outwardly from the spark gap.
Before the spark gap has recovered and can hold the high voltage of another high-energy pulse, the gas between the electrodes must be returned to approximately its initial state or replaced with fresh gas. One way to rapidly restore the density and resistance of the gas in the spark gap is to introduce a purge flow in a flow channel which sweeps the hot residue produced by preceding sparks downstream and introduces a new charge of the gas into the spark gap and adjacent regions. This necessitates controlling the resulting pressure waves to simultaneously restore the density of the fresh purge gas at the spark gap to the original density. While such methods are effective, they can require a great deal of power to circulate the purge gas flow at high pulse repetition frequencies.
Another problem results since the light, heat, and shock waves produced in the spark channel are radiated toward the interior spark gap switch walls. Typically, the exterior spark switch walls are cooled, such as by liquid cooling, which passes through cooling passages within the solid dielectric or on its exterior. Therefore, the large temperature differential between interior and exterior spark switch walls creates significant mechanical stresses in the walls, often resulting in their mechanical failure as the spark gap is operated at high pulse repetition rates.
Conventional approaches to the problem of clearing the spark gap are described in U.S. Pat. Nos. 4,027,187, 4,077,020, 4,237,404, 4,360,763, and 4,563,608.
It is therefore desirable to have a spark gap switch that can operate efficiently and reliably for long periods of time and at higher power levels and pulse repetition frequencies, without requiring significant power input to circulate a purge gas and without subjecting the spark gap switch walls to high thermal/mechanical stresses.